1. Field of the Invention
The present invention relates to couplings between shafts and in particular forms to universal joints and, more particularly to couplings having or seeking to achieve equal instantaneous input shaft and output shaft angular velocities.
2. Description of Related Art
The problem of coupling two rotating shafts operating at an angle to each other has confronted engineers since at least the beginning of the industrial revolution. The “Cardan Joint” developed initially by Cardan in the 16th century is in principle still in use today despite its inherent shortcomings and is found for example in virtually every rear wheel drive vehicle.
An inherent flaw in the design of the simple Cardan Joint is the fact that at any angle between input and output shafts other than 180 degrees, the angular velocity of the output shaft fluctuates sinusoidally relative to that of the input shaft.
Commonly, and as employed again for example in the drive lines of rear wheel drive vehicles, two Cardan Joints are employed, coupling the input and output shafts to an intermediate shaft. By maintaining a parallel alignment between input and output shafts and matching orientations of joint elements, equal angular velocities can be maintained for the input and output shafts with the fluctuations now restricted to the intermediate shaft.
However fluctuating stresses arising from the variations in input and output shaft angular velocities with that of the intermediate shaft are required to be absorbed in the two Cardan Joints. As well it is impossible in many applications and in particular in road vehicles to maintain a strict geometric relationship between input and output shafts giving rise to vibrations, mechanical stresses and power transmission losses.
A partial solution to the problem of maintaining input and output shaft alignment was developed as the so-called “Double Cardan Joint”, often referred to as a Constant Velocity Joint, which is an assembly of two Cardan joints coupled to a short intermediate shaft together with a centering mechanism which constrains both joints to be held in a fixed geometric relationship to each other such that the input and output shafts form equal angles with the intermediate shaft. The major shortcomings of this arrangement reside in the transfer of any axial and radial loads to the centering mechanism resulting in accelerated wear and frictional losses.
Numerous other couplings have been developed to seek to achieve constant angular velocity transfer between shafts. Generally all suffer from being approximate solutions to the strict geometrical constraints of a true constant velocity coupling or achieve an approximation to the geometry at the cost of high wear frictional losses from sliding components.
It is an object of the present invention to address or ameliorate at least one of the above disadvantages or at least provide a useful alternative.